Such a switching device is used to better utilize cable installations that consist of copper lines. In the known process according to the above described Technical Information, four subscribers and their telephones are connected to only one two-wire line, by inserting the system units.
In the known process, the letters "PCM" represent pulse-code-modulation. In this instance, the transmission of the signals takes place after digitalization between the exchange and the subscribers at a transmission rate of 144 kbit/s, as the useful bit rate. The analog voice signals are digitalized at the exchange or at the subscribers, then transmitted through the two-wire line and converted back to analog voice signals at the end of the transmission path. The digitalized voice channels are transmitted between the subscribers and the exchange, or vice versa, through the two-wire line in four channels of 32 kbit/s each. The switching device can also be used for other transmission rates and other subscriber numbers.
In addition to other components, to which analog/digital converters belong in particular, each of the system units contains a hybrid switching unit. In conjunction with a transformer, a hybrid switching unit serves to connect a four-wire system, which contains a transmitting branch and a receiving branch, with a two-wire system, or vice versa. The transformer is located between the two-wire system and the four-wire system.
In addition to the analog voice signals--in the following called "NF" for short--the exchange also transmits metering pulses, which have a frequency of 16 kHz, for example. They are superimposed on the NF. The system unit filters out the metering pulses and transmits them separately through the two-wire line.
Independently of the transmission of the metering pulses to the subscriber, not addressed in this instance, it must be ensured that they do not impair the NF transmission. While the NF level is at about 1 V, the metering pulses are at a level of about 12 V. They must therefore be suppressed by the hybrid switching unit, until they are no longer present at its analog input. Otherwise NF distortions could be produced. Furthermore there is the danger of the hybrid switching unit's components being overmodulated.